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Single-Crystal X-Ray Diffraction of Grunerite up to 25.6 Gpa: a New High-Pressure Clinoamphibole Polymorph

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Single-Crystal X-Ray Diffraction of Grunerite up to 25.6 Gpa: a New High-Pressure Clinoamphibole Polymorph Physics and Chemistry of Minerals (2019) 46:215–227 https://doi.org/10.1007/s00269-018-0999-1 ORIGINAL PAPER Single-crystal X-ray diffraction of grunerite up to 25.6 GPa: a new high-pressure clinoamphibole polymorph Tommy Yong1,2 · Przemyslaw Dera1,2 · Dongzhou Zhang2 Received: 27 May 2018 / Accepted: 23 August 2018 / Published online: 5 September 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract High-pressure single-crystal X-ray diffraction experiments were conducted on natural grunerite crystals with composition (Fe5.237Mg1.646Ca0.061Mn0.051Na0.015Ti0.002Cr0.001K0.001)(Si7.932Al0.083)O22(OH)2, using a synchrotron X-ray source. Grunerite has C2/m symmetry at ambient conditions. The samples were compressed at 298 K in a diamond-anvil cell to a maximum pressure of 25.6(5) GPa. We observe a previously described phase transition from C2/m (α) to P21/m (β) to take place at 7.4(1) GPa, as well as a further transition from P21/m (β) to C2/m (γ) at 19.2(3) GPa. The second-order Birch–Murnaghan 3 equation of state fit to our compressional data, yielded the values V0 = 914.7(7) Å and K0 = 78(1) GPa for α-grunerite, 3 3 V0 = 926(5) Å and K0 = 66(4) GPa for β-grunerite and V0 = 925(27) Å and K0 = 66(13) GPa for γ-grunerite. The β–γ phase transition produces a greater degree of kinking in the double silicate chains of tetrahedra accompanied by a discontinuous change in the a and c unit cell parameters and the monoclinic β angle. At 22.8(4) GPa the O5–O6–O5 kinking angle of the new high-pressure C2/m phase is 137.5(4)°, which is the lowest reported for any monoclinic amphibole. This study is the first structural report to show the existence of three polymorphs within an amphibole group mineral. The high-pressure γ-phase illustrates the parallel structural relations and phase transformation behavior of both monoclinic single and double chain silicates. Keywords Amphibole · Phase transition · High pressure · Single-crystal X-ray diffraction · Diamond anvil cell · Synchrotron source Introduction metamorphic rocks. Amphiboles have such unique chem- istry that they are useful tools as petrogenetic indicators Amphiboles are one of the most important rock-forming (Hirschmann et al. 1994). Structural studies of the amphi- mineral groups found in the Earth’s crust and upper mantle. bole group minerals have been well documented at ambient They are widespread in altered oceanic crust and subduct- conditions, however, despite their ubiquity, relatively few ing slabs in amphibolite, greenschist, and blueschist facies structural studies have been conducted under high-pressure conditions. Previous high-pressure structural studies have been limited to pressures below 10 GPa (Comodi et al. 1991, Electronic supplementary material The online version of this 2010; Zhang et al. 1992; Yang et al. 1998; Welch et al. 2007, article (https ://doi.org/10.1007/s0026 9-018-0999-1) contains 2011; Zanazzi et al. 2010; Nestola et al. 2012), whereas supplementary material, which is available to authorized users. higher pressure studies have only involved spectroscopic * Tommy Yong observations (Iezzi et al. 2006, 2009, 2011; Thompson et al. [email protected] 2016). The cummingtonite-grunerite solid solution is of struc- 1 Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii tural significance and interest as this binary join has three at Mānoa, 1680 East West Road, POST Bldg, Honolulu, different ambient structural phases, orthorhombic Pnma HI 96822, USA anthophyllite (Mg end-member), monoclinic P21/m Mg- 2 Hawaii Institute of Geophysics and Planetology, School rich cummingtonites and monoclinic C2/m grunerites. of Ocean and Earth Science and Technology, University Earlier experiments on the cummingtonite-grunerite solid of Hawaii at Mānoa, 1680 East West Road, POST Bldg, solution series have shown that grunerite [Fe-end member, Honolulu, HI 96822, USA Vol.:(0123456789)1 3 216 Physics and Chemistry of Minerals (2019) 46:215–227 2+ Fe7Si8O22(OH)2] undergoes a phase transition from C2/m as Fe to maintain charge balance, however, the presence 3+ (α) to P21/m (β) with increasing pressure (Yang et al. 1998; of trace amounts of Fe is a possibility. Garnet, chromite, Boffa Ballaran et al. 2000), while P21/m Mg-rich cumming- albite, diopside, scapolite, sphene glass and orthoclase tonites transforms to C2/m at high temperature (Prewitt et al. standards were used. 1970). The chemical formula was calculated based on 23 O The amphibole crystal structure is characterized by dou- atoms as described by Hawthorne and Oberti (2007). This ble chains of silicate tetrahedra which extend along the [001] calculation assumes that (O, OH, F, Cl) = 2 apfu and as no crystallographic direction. A band of octahedrally coordi- F and Cl were detected from the microprobe analysis, we nated cations, designated by sites M1, M2, M3 and M4, link assumed that there were 2 apfu of OH. The results from the adjacent chains of silicate tetrahedra, T1 and T2, along the microprobe analysis are shown in Table 1. a axis. The main difference between the C2/m and P21/m structure, is that the C2/m phase contains one crystallo- Ambient‑pressure X‑ray diffraction graphically distinct O-rotated silicate chain, while the P21/m structure contains two double silicate chains, the S-rotated To characterize the ambient pressure crystal structure of A chain and O-rotated B chain [see Papike and Ross (1970) the sample used in the high-pressure experiments, a euhe- for a description of S- and O-rotated chains]. Furthermore, dral, platelet crystal, approximately 0.15 × 0.09 × 0.02 mm the M4 site cation increases its coordination from 6 to 7 after in size was selected. The crystal was mounted on a Bruker the C2/m–P21/m phase transition. The structure and crystal D8 Venture single crystal diffractometer with a Ag IµS chemical relations of amphibole group minerals have been microfocus source (0.56089 Å) and PHOTON-II CPAD well documented by Papike and Ross (1970), Papike and detector at the University of Hawaii at Manoa’s X-ray Atlas Cameron (1976), Law and Whittaker (1980) and Hawthorne Diffraction Laboratory. The X-ray diffraction data were and Oberti (2007). collected from a θ range of 3.077° to 25.547° with com- Spectroscopic experiments on a synthetic amphibole with pleteness to θ = 19.665. Least-squares structure refinement composition, Na(NaMg)Mg5Si8O22(OH)2 and P21/m ambi- was done with the program SHELXL (Sheldrick 2008). ent symmetry, have indicated a possible existence of a new The initial structure model of grunerite from Finger (1969) high-pressure phase characterized by a C-centered lattice, was used. All atoms were refined using anisotropic atomic with the phase transition likely to occur between 20 and displacement parameters. Full occupancy was assumed for 22 GPa (Iezzi et al. 2006). This new high-pressure phase the M1, M2, M3, T1 and T2 sites, based on the calculated change may be analogous to the structural changes seen chemical formula for our sample. Partial occupancy was in clinopyroxenes with increasing pressure, however, the refined for the M4 site, which is nominally occupied by previous study did not include structure determination. To Fe2+ (Hirschmann et al. 1994). The A site was assumed to shed new light on the nature of the higher-pressure amphi- be unoccupied as stoichiometric grunerite has a vacant A bole phases we reinvestigated the compressional behavior site, despite our crystal having a small amount of Na and of natural grunerite using synchrotron-based single crystal K, which occupies the A site. The small amount of Na and X-ray diffraction experiments. In this study, we report the K in our crystal is equivalent to < 0.19 of an electron per results of a previously unknown phase transition in natural unit cell, which is too small of a charge to be detected by grunerite between 16.3(3) and 19.2(3) GPa from P21/m (β) the difference-Fourier maps in the structure refinement. The to C2/m (γ). structure was refined using the determined chemical formula from the microprobe analysis as a restraint, however, as dif- fraction experiments cannot resolve small compositional Experimental procedures differences, the small amounts of Ti (0.002 apfu) and Cr (0.001 apfu) were ignored. Fe and Mn have similar X-ray Chemical analysis atomic scattering factors, and as such, these atoms were grouped together in the refinement. The M1, M2 and M3 In this study, we used a natural grunerite sample from sites were only occupied by Mg2+ and Fe2+, the M4 site was Moose Mountain Mine, Ontario, Canada with composition occupied by Fe2+ and Ca2+ and the T1 and T2 sites only (Fe5.237Mg1.646Ca0.061Mn0.051Na0.015Ti0.002Cr0.001K0.001) contained Si. The refinement assumed no substitutional dis- 2+ (Si7.932Al0.083)O22(OH)2 determined by wavelength-disper- order of Mg in the M4 site as there is a strong preference sive spectrometry (WDS) using a JEOL Hyperprobe JXA- for Fe2+ and Ca2+ in the M4 site (Hirschmann et al. 1994). 8500F at 15 keV. Composition was determined through the The determined site occupancies for the four M sites are: average of 16 spot analyses on three different single crystals. M1: Fe2+ = 0.731(4), Mg2+ = 0.269; M2: Fe2+ = 0.560(4), The samples were homogenous with no zoning as evidenced Mg2+ = 0.440; M3: Fe2+ = 0.749(6), Mg2+ = 0.251; M4: from the electron backscattered image. All iron was assigned Fe2+ = 0.966(4), Ca2+ = 0.022(4).
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